The surfaces of steel products under corrosive environments, such as automotive parts, are coated with plating films (plating metal layers) such as zinc, nickel, and chromium to reduce corrosion of the steel substrate. Under a mildly corrosive environment, steel is generally coated with zinc plating or the like that provides sacrificial corrosive effect. However, under a harsh corrosive environment such as a strongly acidic atmosphere, simply applying plating with the sacrificial corrosive effect may not sufficiently prevent progress of corrosion of the steel substrate.
Accordingly, a method has been adopted in which a highly corrosion-resistant stainless steels are used in steel products and further the stainless steel as a base material is coated with a plating metal layer (barrier plating metal layer) of a highly corrosion-resistant metal. As an example of such a method, Japanese Patent Application Publication No. 2004-205059 (JP 2004-205059) describes a method of plating a stainless steel substrate, in which a phosphorus-containing nickel film is deposited on the surface of a ferritic or austenitic stainless steel plate by electroless plating and subsequently the phosphorus-containing nickel is diffused into the interior thereof by heat treatment. In accordance with this plating method, although nickel of the phosphorus-containing nickel film (nickel plating layer) crystallizes by the heat treatment, since the nickel plating layer is coated over the surface of the stainless steel, the corrosion resistance of the stainless steel is improved.
However, even if a plating layer such as a nickel plating layer is provided over the surface of the stainless steel as disclosed in JP 2004-205059, the plating layer may corrode if it is exposed to a strongly acidic environment at, for example a pH of 2-3 for a long period.
If corrosion of the plating layer further progresses, corrosion reaches the surface of the stainless steel substrate (base material). At this point, since the stainless steel is a baser metal than the material of the plating layer, when corrosion further progresses, as shown in FIG. 5, the stainless steel falls into a corrosion state with pitting corrosion. When corrosion further progresses in the thickness direction of the stainless steel from such a state, corrosion holes pass through the interior of the stainless steel. As a result, a part made of the stainless steel may lose its original function. In the case that the stainless steel without plating is used, a passivation film of chromium oxide will form. In this case as well, the stainless steel falls into a corrosion state with pitting corrosion similar to FIG. 5.
Further, small holes that extend from the surface of the plating layer of nickel or the like to the interior, which is referred to as pinholes, are slightly formed in such a plating layer. A corrosive liquid such as an acid solution infiltrates via the pinholes. Pitting corrosion may thereby occur in the stainless steel substrate (base material) as described above.